An effective bearing assembly is one which, when in use for supporting a rotating shaft, allows the shaft to rotate as freely as possible, whilst providing the necessary support for the rotating shaft. In addition, the bearing assembly should accommodate and dampen vibration of the shaft. The performance of a bearing assembly becomes more important as the speed of rotation of the shaft being supported increases. The design and characteristics of bearing assemblies for supporting shafts in equipment such as a centrifugal compressor is critical if the shaft is to be allowed to rotate at the very high speed required and if the bearing is to function well over a wide range of speeds of rotation, for example during start-up and shut-down of the rotating equipment. Squeeze-film bearings have been employed for supporting shafts rotating at high speed. The bearing accommodates vibration of the shaft by using a film of fluid, sometimes in combination with one or more centering springs, to dampen the vibration. Examples of hydrostatic bearings are disclosed in U.S. Pat. Nos. 3,121,596, 3,994,541, 4,097,094, 4,392,751 and 3,863,996.
U.S. Pat. No. 2,921,533 discloses a bearing assembly for use in a pump in which, during start-up of the pump, the bearing acts as a hydrodynamic type of bearing while the fluid pressure available is too low for a hydrostatic type of bearing. Thereafter, once the normal fluid working pressure has been obtained, the bearing acts as a hydrostatic bearing.
A significant improvement in high-speed bearing design was made by the introduction of the hydrodynamic shaft bearing with a concentric outer hydrostatic bearing. Such a bearing is disclosed in U.S. Pat. No. 4,365,849. This bearing assembly employs both a hydrodynamic bearing and a hydrostatic squeeze-film bearing. The rotating shaft being supported is located in a cylindrical bore in a cylindrical inner bearing member. The inner bearing member is, in turn, located within a cylindrical bore in an outer bearing member. When in use, fluid is supplied to the cylindrical bore in the outer bearing member by way of passageways extending through the outer bearing member itself. In this way, a hydrostatic squeeze-film bearing is established between the surface of the bore in the outer bearing member and the outer surface of the inner bearing member. Fluid flows from this hydrostatic bearing through passageways in the inner bearing member to the cylindrical bore in which the shaft is located. The fluid entering the bore in the inner bearing member forms a hydrodynamic bearing between the outer surface of the shaft and the surface of the bore in the inner bearing member. It is described in U.S. Pat. No. 4,365,849 that the inner hydrodynamic bearing may be designed so as to optimize the rotational support of the entire bearing system. In this way, the stiffness of the inner hydrodynamic bearing may be maximized to improve rotation and need not be reduced in order to provide a dampening effect to accommodate vibrations set up in the rotation shaft. The dampening effect is achieved using the outer hydrostatic squeeze-film bearing which can be optimized for that function. O-ring seals are provided between the inner and outer bearing members to maintain the pressure of the fluid in the hydrostatic squeeze-film and maintain the performance of the hydrostatic bearing.
The bearing assembly disclosed in U.S. Pat. No. 4,365,849 has achieved considerable commercial success, in particular in applications for supporting shafts rotating at high speed, for example centrifugal compressors. However, it has been found that in order to achieve optimum performance using this bearing assembly, the fluid supply pressure must be significantly higher than other, more conventional, bearing designs. While this in itself does not present a problem in many cases, it would be desirable to provide a bearing assembly which achieves the same high performance, but which does not require such high fluid supply pressures.